Abstract

In this project, a simple single phase gridconnected photovoltaic (PV) inverter topology was implemented, this topology will not inject any lower order harmonics into the grid due to high-frequency pulse width modulation operation. The novel design of inverter that mitigates the lower order harmonics is presented in this project. A proportional-integral (PI) controller are implemented in order to overcome the lower order harmonic distortions. Low total harmonic distortion less than 5% have been achieved. The complete design has been validated with MATLAB software and the overall system operation is observed

Index Terms

filters, inverters, PI controller

INTRODUCTION

Renewable sources of energy such as solar, wind, and
geothermal have gained popularity due to the depletion of
conventional energy sources. Hence, many distributed
generation (DG) systems making use of the renewable
energy sources are being designed and connected to a grid.
How power quality are affected and in what way it can be
reduced are seen in this chapter.

DISTRIBUTED GENERTION

In this project, one such DG system with solar energy as
the source is considered. Single-phase grid tied inverter is
one among types of inverters widely used in photovoltaic
(PV) generation system due to the advantages they offer.
Application of PV as a source of electrical energy showed
a tendency to increase in terms of generation capacity and
in terms of its spread in large areas around the world.
Many aspects trigger the trend; economic, technology and
policy are some among many. The restricted reserve of
fossil fuel sources and followed by the increasing cost of
fossil fuel based electricity generation has motivated the
effort to exploit other alternative energy sources.

In the other hand, the high price of equipment and system
of photovoltaic generation as the main constraint on
implementing this renewable generation system shows
significant reduction during recent years; implicates to
declination of production cost per kW electric from
photovoltaic. The maturity and continuously improved
technology implemented on photovoltaic generation
system that causes the photovoltaic power conversion
more efficient, the typical advantages of PV generation
compared with other electrical generation systems of
renewable energy sources such as its flexibility and
simplicity to build in any places, their dependency from
transportation system are some technical factors causes the
change to this type of renewable energy generation for
electricity is preferred.

HARMONICS

Harmonic is a non-sinusoidal component present in a
complex wave having a frequency of integral multiples of
the fundamental frequency. A complex wave form can be
mathematically resolved using Fourier series as a sum of a
sequence of sinusoids such as f0 + 2f0 +3f0
+4f0+5f0+6f0+7f0+…where f0 is called the fundamental,
multiples of f0 are the harmonics. The basic harmonics
waveforms are shown

ORIGIN OF LOWER ORDER HARMONICS

The dominant causes for the lower order odd harmonics
are the distorted magnetizing current drawn by the
transformer, the inverter dead time, and the semiconductor
device voltage drops. Other factors are the distortion in the
grid voltage itself and the voltage ripple in the dc bus. The
dead-time effect introduces lower order harmonics which
are proportional to the dead time, switching frequency, and
the dc link voltage. The dead-time effect for each leg of the
inverter can be modeled as a square wave error voltage out
of phase with the current at the pole of the leg. The device
drops also will cause a similar effect but the resulting
amount of distortion is smaller compared to that due to the
dead time. Thus, for a single-phase inverter topology
considered, net error voltage is the voltage between the
poles and is out of phase with the primary current of the
transformer.

EFFECTS OF HARMONICS

The detrimental effects of harmonic distortion can be
manifested in many different ways, like as following.

Harmonic filters isolate harmonic current to protect
electrical equipment from damage due to harmonic voltage
distortion. They can also be used to improve power factor.
Harmonic filters require careful application to ensure their
compatibility with the power system in present and future
nonlinear loads. Harmonic filters tend to be relatively large
and can be expensive. Harmonic Filter types include:

1. Passive filters

2. Active filter

PASSIVE FILTERS

Passive harmonic filters are built with a series of passive
components such as resistors, inductors and capacitors. Passive filters are most common and available for all
voltage levels. They are built by combinations of
capacitors, inductors (reactors) and resistors. Often,
passive filters cannot provide optimal harmonic current
reduction without unwanted side effects such as ringing
transient response, unwanted resonances, and
overcompensation. Diode-rectifier loads, typical of
computer power supplies, often require very little 50 Hz
power factor correction capacitance relative to the level of
harmonic currents generated. As such, it is extremely
difficult to provide a passive harmonic filter that does not
overcompensate at 50 Hz, causing a leading displacement
power factor.

Passive filters for harmonic reduction provide low
impedance paths for current harmonics. Thus, the current
harmonics flow into the shunt filters instead of back to
supply. The passive filter consists of series LC filters tuned
for specific harmonics, normally combined with a high
pass filter used to eliminate the rest of the higher-order
current harmonics. The drawbacks with passive filters are
that they are strongly dependent on the system impedance,
which depends on the distribution network configuration
and the loads.

ACTIVE FILTERS

Active filters are those which consist of active components
like Thyristors, IGBTs, MOSFETs, etc. Active filtering
techniques have drawn great attention in recent years.
Active filters are mainly for the purpose of compensating
the transient and harmonic components of load current iL
so that, only fundamental components remain in the grid
current. Active filters are available mainly for low voltage
networks. The active filter uses power electronic switching
to generate harmonic currents that cancel the harmonic
currents from a nonlinear load. By sensing the nonlinear
load harmonic voltages and/or currents, active filters use
either.

PROPOSED SYSTEM

In this paper a single phase grid connected PV
(Photovoltaic) inverter system are considered, the main
objective of the system is to reduce the lower order
harmonics that are induced on the inverter due to dead time
of inverter and transformer core-saturation. These lower
order harmonics will make switching losses and reduce the
efficiency of system, the higher order harmonics cannot
affect the system largely and it can be eliminated by using
passive or active filters. The proposed system consists of
PI (Proportional & Integrator) controller with a passive filters and second order low pass filter are used to reduce
the lower order harmonics

ADVANTAGES

1. The switches are all rated for low voltage which
reduces the cost

2. Lesser component count in the system improves
the overall reliability.

3. It will be a good choice for low-rated PV
inverters of rating less than a kilowatt.

4. The cost of the system is very low.
The THD of the system will be less than 5%.

GRID CONNECTED SINGLE PHASE PV INVERTER

The grid-connected single phase photovoltaic(PV) inverter
consisting of a boost section, a low-voltage single-phase
inverter with an inductive filter, and an isolation
transformer interfacing with the grid is considered. Ideally,
this topology will not inject any lower order harmonics
into the grid due to high-frequency pulse width modulation
operation. A proportional-integral (PI) controller are
implemented to reduce the lower order harmonics

BLOCK DIAGRAM

The topology of the solar inverter system is simple. It
consists of the following stages as shown in Figure.2.

1. A PV module with Maximum Power Point
Tracking (MPPT).

2. A boost converter stage to perform (DC-DC
converter).

3. A low-voltage single-phase H-bridge inverter.
An isolation transformer for interfacing with the
grid.

PHOTOVOLTAIC GENERATION SYSTEM

To analyze the power quality behaviors resulted from
operation of a PV (Photovoltaic) plant in distributed
generation system, a review of the PV generation system
and some aspects that involved in their operation as the
part of the system must be done, they are: the PV module,
PV inverter and the module-inverter configuration, the PV
plant-grid interaction and the atmospheric condition. The
main components of a PV generation plant are the PV
modules and the PV inverters. The PV module is used as
energy conversion equipment, converting the light energy
to electrical form of the dc voltage and current. The
conversion involves interaction process of the light,
thermal and electrical parameters in a photovoltaic
material. The PV inverter is then used to convert the dc to
ac power to be used by consumer or to be connected to the
grid.

PHOTOVOLTAIC MODULE

Photovoltaic module is set up from formation of
photovoltaic cells that convert the energy in the light to
electric power. The modules are then arranged in both
series and parallel configuration as a photovoltaic array to
reach the voltage and current requirement. Photovoltaic
cells are semiconductor devices that draws non-linear
characteristics between output current (I) and voltage (V)
on their terminal. In a condition when the light shapes their
surface, the light generations current are produced in
proportional to the light intensity, in the same time a dc
voltage is generated. If the generated voltage is high
enough, the solar cell current drops extremely, similar to
the behavior of the diode as seen on the knee point of the
diode’s characteristic. From these behaviors, based on
circuit perspective, photovoltaic cell can be modeled as
configuration of current sources in parallel with some
diodes, serial and parallel resistors are then added to
present the voltage and current losses during cell
operation. A photovoltaic cells model called the single
diode model is widely used as shown in Figure.3

Ipv and Vpv are the current and voltage of the PV cell. Iph is
the light generating current, its value depends on irradiance
and the physical dimension of photovoltaic cell, Io is the
diode dark saturation current. This single diode model of
PV cell can be described using the follows mathematical
model to form the cells

The Boltzmann constant = 1.3807 x 10-23 JK-1 and
electric charge = 1.6022 x 10-19 C are presented by k and
q. Rse and Rsh are the representation of the parasitic series
and shunt resistances that associated with real solar cells in
operation condition. A0 is the diode ideality (quality)
factor; its value is taken between 1 and 2, A0 = 1 indicates
that diode behavior of cell is dominated by recombination
in the quasi-neutral regions and A0 = 2 indicates that
recombination in the depletion region dominates. ISCR is
short circuit current of the cell on 1000 W/m2 and 25 OC
of temperature. K1 is the short circuit temperature
coefficient at ISCR. T and Ga are cell temperature and
irradiance on cell surface. I0r is the cell saturation current at
reference temperature Tr. EGO is band gap energy.

MAXIMUM POWER POINT TRACKING

The efficiency of a PV plant is affected mainly by three
factors: the efficiency of the PV panel (in commercial PV
panels it is between 8-15%, the efficiency of the inverter
95-98 % and the efficiency of the maximum power point
tracking (MPPT) algorithm (which is over 98%. Improving
the efficiency of the PV panel and the inverter is not easy
as it depends on the technology available, it may require
better components, which can increase drastically the cost
of the installation. MPPT algorithms are necessary because
PV arrays have a nonlinear voltage-current characteristic
with a unique point where the power produced is
maximum. The various methods are used for improving the efficiency of MPPT algorithm, for these P&O (Perturb
& Observe) used.

The Perturbation and Observation algorithm is broadly
used due to it is simple and easy to construct. According to
the flowchart illustrated in Figure 4.P&O method basically
increases or decreases (the perturbation stage) the
controller reference voltage by a step size noted as C,
hence the PV source terminal voltage, and
subsequently estimates the power difference between the
present PV power and that before the perturbation
(observation stage). If a positive PV power difference
is obtained that means the PV power is increased and
the tracking is in the right direction, the perturbation
direction will be carried on (increase or decrease). On the
contrary if power difference is negative, a power
reduction is caused due to the perturbation, so the
direction of perturbation should be reversed.

As a result of continuous perturbation, P&O algorithm
may not stop at the desired MPP voltage but oscillates
around it, causing PV power loss. One way to
minimize the tracking oscillation is to reduce the
perturbation voltage step. As a consequence of that, in
the constant irradiance state the PV power curve will
be smooth with very small ripples. However, the
tracking speed will detract causing more power lose
and losing the ability to track certainly at the rapid
atmospheric changes. Even though choosing a large
perturbation step will result in achieving fast tracking
response at the suddenly atmospheric changes, in the
steady state the mentioned oscillation will be quite
considerable.

BOOST CONVERTER STAGE

A boost converter (step-up converter) is a DC-to-DC
power converter with an output voltage greater than its
input voltage. It is a class of switched-mode power supply
(SMPS) containing at least two semiconductor switches (a
diode and a transistor) and at least one energy storage
element, a capacitor, inductor, or the two in combination.
Filters made of capacitors (sometimes in combination with
inductors) are normally added to the output of the
converter to reduce output voltage ripple. The switch is
typically a MOSFET, IGBT, or BJT.

Soft switching can mitigate some of the mechanisms of
switching loss and possibly reduce the generation of EMI.
Semiconductor devices are switched on or off at the zero
crossing of their voltage and current waveforms. For this
project a ZVZCS methods are adopted as shown in Fig.5.
The zero-voltage zero-current switching (ZVZCS) PWM
converters are derived from the full-bridge phase-shifted
zero-voltage (FB-PS-ZVS) PWM converters.

H-BRIDGE CONFIGURATION

An H-bridge or full bridge converter is a switching
configuration composed of four switches in an
arrangement that resembles an H. By controlling different
switches in the bridge, a positive, negative, or zeropotential
voltage can be placed across a load. When this
load is a motor, these states correspond to forward, reverse,
and off. The use of an H-bridge configuration to drive a
load is shown in Fig.6. The H-bridge circuit consists of
four switches corresponding to high side left, high side
right, low side left, and low side right. There are four
possible switch positions that can be used to obtain
voltages across the load. These positions are outlined in
Table .1. The harmonic table for switched mode inverter
are also shown in appendix (B).

Note that all other possibilities are omitted, as they would
short circuit power to ground, potentially causing damage
to the device or rapidly depleting the power supply. The
switches used to implement an H-bridge can be mechanical
or built from solid state transistors. Selection of the proper
switches varies greatly. The use of P-channel MOSFETs
on the high side and N-channel MOSFETs on the low side
is easier, but using all N-channel MOSFETs and a FET
driver, lower “on” resistance can be obtained resulting in
reduced power loss. The use of all N-channel MOSFETs
requires a driver, since in order to turn on a high side Nchannel
MOSFET, there must be a voltage higher than the
switching voltage. This difficulty is often overcome by
driver circuits capable of charging an external capacitor to
create additional potential.

ISOLATION TRANSFORMER

Isolation transformers offer an effective means of meeting
the requirements of domestic and international safety
standards for electronic equipment. In the United States,
for example, such standards are set by the Occupational
Safety and Health Administration (OSHA), with product testing performed according to appointed laboratories,
such as Underwriters Laboratories (UL). Throughout
Europe, safety standards are established by the
International Electro technical Commission (IEC), with
testing performed by the laboratories of individual member
nations, such as the Verband Deutscher Electro techniker
(VDE) in Germany. Isolation transformers enable a variety
of electronic systems to meet safety requirements. Such
systems include medical diagnostic equipment, computer
systems, and telecommunications equipment.

CONCLUSION

A grid connected single phase PV inverter has been
proposed,for ensuring high quality of the current injected
to the grid. The PI controller was implemented to reduce
the dead time of inverter and lower order harmonics. By
using MATLAB simulation it was observed that the
overall THD value of the PV system is reduced less than
5%